The clunk may just be the glow plug relay being triggered rather than the starter solenoid.

You don't have the terminal block (that vwnate described) in your 240D.
To test your solenoid, connect a remote starter switch to terminal 30 and terminal 50 (the purple wire and red wire on your solenoid) or jumper these two terminals.
See instruction 3b of the FSM job 05-210.

The OM617A equipped W116's, W123's and W126's have the terminal block vwnate described, as shown in step 2b of FSM job 05-210 for these vehicles.

If the solenoid tests okay, other possibilities:

1) Poor quality repairs to previously heat damaged wiring harness to the starter solenoid.
Looks like a used solenoid harness was spliced into the original harness.
I suspect poor non-soldered connections under the red shrink-wrap wire protectors.
I also suspect a poor trigger wire connection, (small purple wire on starter solenoid)... non-soldered crimp on connector repairs can fail.

Now maybe the time to re-repair, make or buy a new harness.

2) Worn ignition switch electrical contacts, intermittently causing no power to small purple wire on your starter solenoid when your turn the ignition key to the start position.

Disconnect negative cable, remove small purple trigger wire at solenoid, connect DVM between purple wire and small screw terminal, reconnect negative cable.
Check voltage at purple wire with DVM, while a buddy tries to start engine with key.
Voltage should be zero at all key positions except the start position.

3) Bad/poor ground strap/connection from engine to body side member.
Item 83 on the diagram in the link.

4) Bad/poor ground strap connection from negative battery to body.
For 3) and 4) remove straps, use extra-fine grit sandpaper to expose fresh shiny metal an all contact surfaces, re-connect the straps ... then cover the connections with dielectric grease.
Replace with new (larger capacity) ground straps if necessary.

Intermittent electrical problems are usually corroded wires or connections -- due to heat, age and flexure.

Best of luck and let us know what fixes the problem.

Thanks Alec.

I'm fairly certain that the clunk is the the starter trying to get power and not that glow relay. That takes 25 seconds for me to hear. I've heard the dead clunk many times on my old Land Rovers. I know the sound well.

The guy I bought it from said the starter was rebuilt. But I'm curious about that wiring since it's wrapped up in electrical tape.

Does Mercedes sell a starter wiring harness or would I have to harvest one?

The starter harness appears to be item 203 in the diagram on the link in post 3 of this thread.
It shows as NLA, so I guess you need to either harvest a used harness, or built yourself a new one.
Use the old one as a template.
Take photos of the routing before removal to aid in installing the replacement.

One end of the harness appears to plug into the alternator.
The spade connectors can get loose causing intermittent electrical contact...and poor charging of the battery.
That end of your harness may need refreshing/cleaning/replacing.

Here is where I am..

http://i632.photobucket.com/albums/u...ps6wpzky0v.jpg

http://i632.photobucket.com/albums/u...psaj57oznv.jpg

http://i632.photobucket.com/albums/u...ps4azdjzas.jpg

http://i632.photobucket.com/albums/u...psdys8xaga.jpg

Not seeing anything alarming going on with the wiring. Maybe a little bit of crusting on the purple wire.

Starter Diagnosis & Repair
 

Copy and print out reply # 31 as Alec wrote a good, detailed post .

I like your super clear pictures ! I only have an old iPod and when I get close, it gets fuzzy and looses clarity .

Anyway , look at your solenoid, there's only three connections on it, use a nickle to jumper the bog one on top and the small one @ 9:00 the next time you hear a clunk, if it doesn't begin to spin and the battery has over 10VDC the starter needs to come off for service .

Agreed, many aftermarket solenoids are crap, why I only use original BOSH or sometimes NAPA's ECHLIN brand as OEM's use Echlin starter solenoids .

BE CAREFUL when jumpering the starter ! you can burn your wrist and if the car's in gear it'll lurch .

I don't know if this typ of solenoid can be taken apart but many can and you can polish (NEVER sand/abrade electrical contacts !) the contacts and wire brush the plunger and use steel wool to clean the inside housing where the plunger slides, make it all clean and smooth, use very little short fiber grease to prevent rust, it needs no lubrication .

Keep at it, I'm betting it's a combination of little things and you'll eventually get into that sub harness, plenty of Junked Mercedes' you can cheaply strip out long pieces of the correct red and purple wires to make up a new un cut starter / charging sub harness .

New information

It happened again at work. I was getting that 'dead click'.. This time I got a co-worker to check where the sound from coming from. It was coming from the glow plug relay box. Everytime I hit the key it just clicks from in there.

We pop started it again and it fired up fine. Then shut it off and it started just fine.

Is this a relay problem? Or something else in there?

Maybe failing ignition switch?

The clunk of the glowplug relay is normal.
Usually you don't hear it because the sound is masked by the starter motor being engaged.
On an automatic, if you put it in gear and try to start the car, the car won't start...but you'll hear the clunk of the glowplug relay as it activates.

Did you try jumping the solenoid when you have the no start situation?

I think you should check the alternator plug end of the solenoid harness.
My 83 W126 300SD has had that triple spade plug vibrate loose...this caused a poor charging situation, and often the battery didn't have enough capacity to start the car.
A rebuilt Bosch alternator was installed but the battery would still randomly go flat...causing a no start situation.
It took me a few weeks of almost daily jockeying of the fresh battery from the W116 to the W126, and the dead battery to the charger, before I finally solved that problem.
Poor connections in starting and charging circuits can make these diesels very hard to start.

I think you may have an intermittent open in your charging circuit.
To test: when the car is running, hook up your DVM to the positive and negative battery terminals...you should be reading over 13 volts.
Carefully reach behind the alternator and jiggle the harness.
If you have a poor connection the voltage will fluctuate.

Before working on the plug to fix the connection issue (if it exists) disconnect the negative cable...you don't want a shower of sparks!!
Unplug the 3 spade alternator harness plug, clean the contacts with electric contact cleaner.
If the connectors don't grip the spades on the alternator well, you can release them from the plastic connector, and snug up the grip a bit with a small pair of needle-nose pliers.
(Press on the tab that locks the wire in the connector with a small screwdriver to release the wire from the plug.)
Then plug it back in, reconnect the negative cable and try to start the car.

Hopefully this will fix your problem.

Looking closely at the first photo, the crimped connector on the the positive battery cable attached to the top post of the solenoid looks like it has a bit of corrosion on the surface.
It may help to disconnect that cable and clean it with electric contact cleaner to assure you have a very good electrical connection.
(Many times there is hidden corrosion under the plastic insulation.)
Remember, first disconnect the negative cable from the battery.

Keep at it, do the free, inexpensive diagnostic tests first.
Checking voltage drops, electric contact cleaner, new wires, and new connectors are much less than a new or rebuilt starter.

You will get it sorted.:)

Thanks again Alec.

I don't think I have a charging or battery problem. Because I can pop star the car, let it run for 30 seconds, shut it off then start it over and over again without any weak cranking or lack of good power to the starter.

Next time it doesn't want to crank I'm going to try to jump the solenoid. I'm willing to bet it will start fine.

Everything I've been researching sounds like a ignition switch going bad. I'm thinking it's getting to the point that every once in awhile it doesn't want to engage the starter. This 240 has seen some use with almost 290,000 miles on it, so I'd guess its probably on it's original ignition switch.

Agreed, next time you have a no start situation, if you jump the solenoid and it starts...then you most likely have an ignition switch problem (#2 on Post 31).

Any open (or excessive resistance ) in the circuit will cause a failure to start.
At this point, you still could have scenario 1, 3 or 4 from Post 31.

Please report back what cures this no start problem.
"Inquiring minds want to know".:)

Today at the bank I had another no crank. So I figured ''No biggie, I'll just jump the starter with a screwdriver''

Well I got the screwdriver down in there, hit the two posts together and I got the same clunk sound from the glow plug relay just like I would if I would turn the key. It did this for maybe a dozen times and finally the starter came to life and started.

I'm all confused now.

Nothing confusing here : the solenoid, even new, is KAPUT .

Either pony up for a BOSCH branded rebuilt starter or buy a brandy new BOSCH or Echlin (NAPA) but NO OTHER BRAND solenoid and install it .

The starter will have to come off the engine .

How many times in this thread alone has the solenoid been diagnosed as the issue? Would be nice if folks would listen sometimes!

The bottom line is that if power is getting to the solenoid and has enough current to cause it to "clunk", you can rule out the ignition switch and the NSS. Go straight for the solenoid and/or starter at that point. The motors themselves rarely fail unless they're very old, but the solenoids are another story!

It could be a bad solenoid but I wouldn't be so sure that's it. It could also be other things.

Jumping the solenoid with a clunk and nothing happens could be:

1. worn brushes/ dead spot on commutator due to worn bushings such that
the commutator is no longer concentric. Try wacking the starter with a 1/2" breaker bar and try jumping the solenoid again.

2. Bad electrical connection to starter and grounds, or bad connections at ground strap, loose or missing starter mounting bolts. This could cause solenoid to not develop full power.

Whoa guys.

I'm not getting a clunk from the starter solenoid. That clunk I hear when I hit the key or try to jump the starter is coming from the glow plug relay box. I figured that out when I finally had someone put their ear in the engine bay when this happens.

That's what I think it's the ignition switch. It's probably gummed up in there from being over 285,000 miles old.

Something like this:

https://dq4zp01npifg0.cloudfront.net...s/IMG_9968.jpg